Fasting, or anything else you do with a wrong motive (You aren’t going to get God to do something by fasting. Fasting helps you hear God better.)

Fasting to show off to others, to show you’re better than others is empty, worthless, and making yourself a god in your own life.

Paul speaks very strongly here against asceticism. Asceticism means severe self-discipline and avoidance of all forms of indulgence, typically for religious reasons. The people of Colossae were being caught up in a false teaching that basically said if you deny yourself that is what saves you.

This is very similar to the chapters in Isaiah that talk about false fasting. Showing off what you do to be above others is idolatry of self.